Yes, please. If that extension shows only the LilyPond source code of
snippets but the normal page view and the printable view show these
snippets processed and converted to PNG, it surely means that LilyPond
is not called to process these snippets from Extension:Collection.

By the way, for a decent printing quality, the PDF generation should
not be left to the user browser but should be done on the server side
(using PDF snippets generated by LilyPond), like whatopenstreetmap.org does. This regards standard printable view as well
as the extension Collection.

Best,John

thank you very much! with the help of your analysis, i now filed a bug to this collection, hoping it will be fixed in the near future.